Natural Light Signals and Phototransduction

The first step in processing signals from the outer world is to transduce them into membrane potential changes in receptor cells at the periphery of the nervous system. In the case of light, this process is called phototransduction . Light is absorbed in photoreceptors by specialized molecules called rhodopsins that initiate a biochemical cascade of reactions resulting in a photocurrent across the membrane of the photoreceptor that leads either to depolarization or hyperpolarization of the membrane potential (depending on the photoreceptor type) and synaptic release of neurotransmitter onto postsynaptic cells. Before describing in more detail these mechanisms, we need to describe the most salient properties of light in the natural world as these have an important impact on its processing by the visual system.